This invention relates generally to air traffic control systems and more particularly to a method and apparatus for determining the beginning and end of a change of aircraft vertical mode of flight.
As is known in the art, air traffic control (ATC) systems promote the safe, orderly and expeditious flow of aircraft traffic. Safety is principally a matter of preventing collisions with other aircraft, obstructions, and the ground, assisting aircraft in avoiding hazardous weather, assuring that aircraft do not operate in airspace where operations are prohibited, and assisting aircraft in distress.
As is also known, ATC systems employ information from both ground based radar and aircraft based transponders to indicate the horizontal and vertical position of one or more aircraft. Aircraft can include a so-called mode C transponder. The aircraft based Mode C transponder, upon interrogation by an ATC system, transmits information to the ATC about the altitude of the aircraft. The altitude determining method of existing ATC systems uses both Mode C data from the aircraft transponder as well as information from a multi-radar tracker (MRT) process. Data from the Mode C transponder and from the MRT are processed by an altitude post processor (APP) method that further enhances the vertical mode of flight (MOF) determination, the determination being that of level or non-level flight.
Existing Mode C data has a resolution of 100 feet, due in part to low resolution of the received Mode C transponder data combined with further limitation of the existing APP processing method. The 100 foot resolution resembles a step function with 100 foot reported altitude jumps as the altitude crosses resolution boundaries. This resolution limit and finite altitude jumps result in both a delay time in the determination of the start or stop of an aircraft altitude change and a relatively high level of uncertainty as to the instantaneous aircraft rate of ascent or decent.
Due to the 100 foot altitude uncertainty, it is necessary to set a minimum limit of 200 feet before an altitude change can be reported by the conventional APP method. It is also necessary to delay altitude change determinations so as to filter out the instantaneous 100 foot uncertainty. Data with this uncertainty is presented to existing Conflict Alert (CA) and Minimum Safe Altitude Warning (MSAW) systems, which are utilized in existing ATC systems. The altitude determination time delay and the altitude uncertainty cause the CA and the (MSAW) systems to miss some real aircraft position conflicts and also to falsely declare some such conflicts. The safety of aircraft monitored by such ATC systems is thereby compromised.
The conventional APP method indicates a change from level to non-level mode of flight (ascending or descending) if all of the following conditions are true: (1) the altitude rate received from the MRT is non-zero and the previous altitude rate computed by the APP was zero; (2) the difference between the current and reference altitude is greater than 100 feet and the difference between their times is at least 3 seconds; and (3) the initial altitude rate, set to ninety percent of the reference speed below, is greater than two hundred feet per minute, where:
xe2x80x83reference speed=(altitude reported by Mode Cxe2x88x92reference altitude)divided by(altitude report timexe2x88x92reference altitude time).
It should be noted that the reference altitude is set at the time the aircraft is first detected and it remains fixed until a non-level flight condition is declared or 65 seconds have elapsed, whichever occurs first, at which time the reference altitude is reset to the current altitude. The reason for this is that if the vertical speed of the aircraft is slow (e.g. 200 ft/min), the vertical distance moved per radar scan (e.g. 5 seconds) is less than 100 ft. Therefore, one needs a reference altitude with which future altitude reports are compared to discern a difference of at least 200 ft. considering the fact that 100-ft. scan-to-scan differences do not indicate vertical motion.
The conventional APP method indicates a change to a level flight track if all of the following conditions are true: (1) the altitude rate received from the MRT is less than or equal to 10xe2x88x926 nautical miles per second; (2) the level altitude reference (LAR) has not been updated for at least the last sixty five seconds. The LAR is a reference altitude that is used to determine when an aircraft resumes level flight. If the vertical speed is slow, it is not possible to determine from scan-to-scan when vertical motion ceases, again because of the 100-ft uncertainty. The LAR is not updated as long as the reported altitude does not change by more than 100 ft. If this condition persists for at least 65 seconds, it constitutes one of the necessary conditions for declaring level flight.
The performance of the aircraft trackers associated with present ATC systems is limited in vertical MOF determination accuracy by the 100 foot resolution and the noisy output nature of the altitude data presented by the Mode C transponder. Due in part to the low quality of this data, trackers have relatively long time delays in their ability to estimate or otherwise determine a change of vertical MOF. A large time delay in the determination of a change of aircraft vertical motion causes the ATC to require large aircraft separations via existing Conflict Alert (CA) methods. Such relatively long time delays can compromise aircraft safety as airspace become increasingly crowded.
It would therefore be desirable to provide a system which more rapidly estimates or otherwise determines the time at which an aircraft mode of flight changes from level to non-level and vise-versa.
In accordance with the present invention, a method for determining when a moving object""s vertical mode of flight changes between level and non-level includes calculating a vertical motion indicator (VMI) value that indicates that the moving object""s vertical mode of flight is either level or non-level, and utilizing the VMI value to initiate and terminate vertical-motion tracks.
With this particular arrangement, the VMI value gives improved vertical mode of flight (MOF) determination accuracy and a more rapid determination of the time of a change in vertical MOF. The method can further include utilizing the VMI value in a modified APP method by providing the VMI value to a processor, providing multi-radar tracking data to the processor, providing altimeter altitude data values to the processor, and processing these three values in the processor to track the moving object""s vertical MOF. The combination of the three data inputs to the modified APP method gives a more accurate and more rapid determination of the altitude and the vertical mode of flight.
In accordance with another aspect of this invention, an apparatus for tracking when a moving object""s vertical mode of flight changes between level and non-level, includes a vertical motion detector (VMD) processor that computes a vertical motion indicator (VMI) value. The VMI value indicates that the moving object""s vertical mode of flight is either level or non-level. The apparatus also includes a flight mode processor that utilizes the VMI value to estimate the moving object""s vertical mode of flight.
With this particular arrangement, the VMD processor provides a VMI value that gives improved MOF determination accuracy to yield a more rapid determination of the vertical mode of flight. In one embodiment, the apparatus can include the flight mode processor that includes a non-level mode processor and a level mode processor. The non-level mode processor utilizes the VMI value, multi-radar tracking data, and altimeter altitude data to determine or otherwise estimate a non-level mode of flight of the moving object. The level mode processor utilizes the VMI value, the multi-radar tracking data, and the altimeter altitude data to determine or otherwise estimate a level mode of flight of the moving object. The modified APP method is implemented by the non-level mode processor and the level mode processor. The non-level mode processor can rapidly determine or otherwise estimate a change from level to non-level flight, such information being used to avoid aircraft position conflicts. The level mode processor can determine when the aircraft changes from non-level to level flight, avoiding other aircraft conflicts.